Trump's Legal Challenges to the California Car Deal

Prompting rage by President Trump, California and several carmakers entered into a voluntary agreement on carbon emissions from new cars that blew past the administration's efforts to repeal existing federal requirements. Last week, the Trump administration slapped back at California. Although there's been a lot of editorializing about that response, I've seen very little about the legal dimensions of the administration's actions. I'd like to shed a little bit of light on those.